1. Bring a large pot of salted water to a boil. Add the pasta and cook as the label directs, then drain. Reserve a cup of boiled pasta water.
2. Meanwhile, melt 2 tablespoons butter in a large skillet over medium-high heat. Add the onion and cook until slightly soft, about 2 minutes. Add the mushrooms and cook, stirring, until they begin to brown, about 2 minutes.
3. Add the remaining 1 tablespoon butter, the chicken, our, chili akes, salt and pepper. Cook, stirring, until the chicken browns, about 3 - 5 minutes.
4. Add the pasta water and bring to a gentle simmer; cook until thickened, about 5 minutes. Stir in the cream and season with salt and pepper. Continue simmering gently until the chicken is cooked through, about 5 more minutes.
5. Divide the spaghetti among plates. Top with the chicken stroganoff and white sauce. Grate cheese and sprinkle thyme leaves. Serve warm.

Enjoy your meal!